Composite sandwich structures are used extensively in aerospace, automotive, and marine applications for primary and secondary structure. Standard sandwich structures include a foam core or honeycomb core and outer layers, called skins or face sheets, that usually are adhesively bonded to the core. The face sheets typically are fiber-reinforced organic matrix resin composites, having, fiberglass, carbon, ceramic, or graphite fibers and a thermosetting or thermoplastic matrix resin. The face sheets carry the applied loads, and the core transfers loads from one face sheet to the other, or the core also absorbs a portion of the applied loads. In either case, it is important that all layers maintain their connection to one another. Sandwich structure and sandwich structure for noise suppression and other applications are described in U.S. Pat. No. 5,445,861, which I incorporate by reference.
Foster-Miller has been active in basic Z-pin research. U.S. Pat. No. 5,186,776 describes a technique for adding Z-pin reinforcement to composite laminates. A dispensing needle vibrating at ultrasonic frequency heats and softens the matrix and penetrates the laminate, moving the laminate fibers aside. The needle inserts a reinforcing fiber into the laminate. The needle is withdrawn, allowing the matrix to cool around the composite. U.S. Pat. No. 4,808,461 describes a structure for localized reinforcement of composite structure including a body of thermally decomposable material that has substantially opposed surfaces, a plurality of reinforcing elements in the body that extend generally perpendicular to one body surface, and pressure intensifying structure on the other opposed body surface for applying driving force to the reinforcing elements for insertion into the composite structure as the body is subjected to elevated temperature and decomposes. I incorporate U.S. Pat. Nos. 4,808,461 and 5,186,776 by reference.
In U.S. patent application Ser. No. 08/582,297 entitled "Pin-Reinforced Sandwich Structure," which I incorporate by reference, I described a method of forming a pin-reinforced foam core sandwich structure including (i) positioning first and second face sheets of uncured fiber-reinforced resin (i.e., prepreg or B-stage thermoset) about a foam core having at least one compressible sublayer and a plurality of Z-pins spanning the foam between the face sheets and (ii) inserting the Z-pins into the face sheets during autoclave curing of the face sheet resin. During autoclave curing, the compressible sublayer is crushed and the Z-pins sink into one or both of the face sheets to form the pin-reinforced foam core sandwich structure. I also described column core structure made by removing at least some of the foam core by dissolving, eroding, melting, drilling, or the like to leave a gap between the face sheets. This structure (1) resists distortion and separation between layers, in particular, separation of the face sheets from the foam core; (2) maintains high structural integrity; (3) resists crack propagation; and (4) easily accommodates the removal of portions of foam core, as required by specific applications.
The foam core generally includes a high density foam sublayer, and at least one low density foam sublayer. The preferred arrangement includes a first and second low density foam sublayer, one placed on each side of the high density sublayer. The plurality of Z-pins are placed throughout the foam core in a regular array normal to the surface or slightly off-normal at an areal density of about 0.375 to 1.50% or higher, as appropriate, extending from the outer surface of the first low density foam sublayer through to the outer surface of the second low density foam sublayer. Expressed in different terms, we use 40-50 pins/in.sup.2 in most applications. Preferably, the foam sublayers are polyimide or polystyrene, the Z-pins are stainless steel or graphite, and the face sheets are partially cured thermosetting fiber/resin or thermoplastic composite materials.
Fastening composite structure, especially to metal, is troublesome. The fasteners concentrate strain by being the principle load transfer paths. The holes cut through the composite either cut the reinforcing fibers or force unusual tow placements during layup. Strain concentration is particularly a concern in the "zero" plys where the matrix must carry the load from the fastener to the fibers in that ply. A joint that reduced the strain concentration and retained the integrity of the reinforcing fibers in the composite would provide a significant advantage. The joint of the present invention is promising for composite-metal joints of modest strength.
Fastening also is troublesome because it often requires altering the composite thickness in the edge band to accommodate bolt lengths. The layup design is made more complicated and the joint has reduced strength because of the reduced thickness (i.e. "gage").
Often composites will be adhesively bonded to the metal, but such bonds are unsatisfactory in many shock or vibration environments.